Discrete level detector

ABSTRACT

A LEVEL INDICATOR FOR INDICATING THE PRESENCE OR ABSENCE OF A FLUENT MATERIAL AT A DISCRETE POSITION. AN ELONGATED SENSING ELEMENT IS PROVIDED WHICH IS OPERATIVE IN THE PRESENCE OF SURROUNDING PRESSURE TO ACTUATE ONE OR MORE REDUCTANT CONTACTS INTEGRALLY FORMED THEREWITH AND WHICH IS ISOLATED FROM THE WORKING ENVIRONMENT.   D R A W I N G

73/301 ZOO/61.21

3,153,342 10/1964 3,550,447 12/1970 Beresic FOREIGN PATENTS ORAPPLICATIONS 1 1 DISCRETE LEVEL DETECTQR 75 inventors: Albert D.Ehrenfried; Norton T.

Pierce, both of Concord, Mass.

[73] Assignee: Metritape, Inc., West Concord,

Mass.

Primary Examiner-Louis J. Capozi Filed! 1970 Assistant Examiner-Denis E.Corr Attorney-Joseph Weingarten [21] Appl. No.: 22,204

[52] US. 73/301, ZOO/61.21, 200/85 R,

7 340/246 A level indicator for mdicatmg the presence or absence [51]Int. G01f 23/18 of a fluent material at a discrete position. Anelongated sensing element is provided which is operative in the [58]Field of 73/301; ZOO/61.2,

presence of surrounding pressure to actuate one or 200/6121 85; 340/244246 more redundant contacts integrally formed therewith and which isisolated from the working environment.

References Cited UNITED STATES PATENTS 9 Claims, 10 Drawing Figures3,583,221 Ehrenfried............................ 73/301 PATENTED3,783,689

SHEET 1 BF 3 INVEYIORS ALBERT D. EHRENFRIED TON T PI E PATENTED JAN 74SHEET 3 0f 3 !.\\'E.\"I'()RS ALBERT D EHRENFR/ED NORTON 7T PIERCE iiz T0ix 1Y5 Fig. 8.

1 DISCRETE LEVEL DETECTOR FIELD OF INVENTION This invention relates tolevel detectors and more particularly to discrete level detectors whichare electrically sensitive to surrounding pressure of a fluent materialand which are isolated from the working environment.

BACKGROUND OF THE INVENTION Discrete level sensing devices capable ofdetecting the presence or absence of fluent material at a selectedposition are widely employed in the measurement and control of materialsstored in bins, silos or other suitable vessels. Such level detectorsare employed, for example, to control filling and draining operations ofdry bulk or liquid material in a storage vessel, one such detector beingplaced at the top of the vessel to indicate a high level condition, andanother detector being placed at the bottom of the storage vessel toindicate a low level condition. Discrete level detectors of conventionalconstruction suffer a number of deficiencies which seriously limit theirusefulness in manyoperating environments. For use in liquid materials, afloat actuated switch is often employed. However, such float mechanismsoften stick due to corrosion and material build-up, especially inslurries and sewage, which limits the utility of such devices.

Capacitance probes and photoelectric sensors have been employed in bothliquid and dry solid materials for level indication. Capacitance sensorsare usually difficult to maintain in adjustment and are subject to driftwith passage of time. In addition, deposits of material on a capacitanceprobe will falsely indicate the presence of material. In like mannerphotoelectric devices are affected by the build-up or deposit of residueand require continual cleaning and maintenance to maintain proper systemoperation.

To indicate discrete level in dry granular solids, a known sensoremploys a rotating paddle wheel which is free to rotate when material isabsent, but which is prevented from rotation in the presence of materialsurrounding the paddle. Such rotatable paddles are commonly mounted inthe side wall of the bin or other storage vessel. As material iswithdrawn from the bin, a shearing action takes place between the massof material and the wall of the bin, and shearing forces of considerablemagnitude can occur with resulting damage or destruction to the sensingelement. Capacitance probes and photoelectric devices similarly sufferthe damaging effects of such shearing forces. A further disadvantage ofpaddle type indicators is that they require a rotating shaft to bemounted through the wall of the storage vessel. Dust and othercontaminants can penetrate the bearings and effect the electricaloperation of such indicators or increase bearing friction tothe point ofpreventing paddle rotation and thereby preventing proper levelindication.

Discrete level indicators of conventional design are generallymechanically rigid in order to withstand the often considerable forcesexperienced in an operating environment. The construction of suchconventional indicators required to maintain a structurally sound deviceoften detracts from the electrical sensitivity of such devices, andattempts to increase the electrical sensitivity thereof usuallyresult inloss of mechanical strength. In addition, conventional indicators arenot easily sealed or isolated from the working environment, as a resultof which such indicators are sensitive to contamination, such as dust ormoisture, from the environment. Moreover, many conventional levelindicators require ancillary linkages to couple the sensing element tothe stored material being sensed. Such linkages can limit the accuracyof level determination and can become clogged with the material in whichthey must 0perate.

SUMMARY OF INVENTION In accordance with the present invention, anelectrically sensitive and mechanically rugged discrete level detectoris provided which is electrically isolated from the environment andwhich is capable of actuation at different points along its length. Thenovel detector includes a plurality of electrically parallel redundantcontacts which can be provided by a continuous manufacturing process toprovide sensors of various lengths and which can be employed to providemultiple zone systems for the discrete indication of material level bymeans of a single sensor structure. The detector 'constructed accordingto the invention essentially has no moving parts and requires noadditional structure or linkages to provide actuation. Actuation of thenovel detector is provided by the force of the material being sensed. Vv

The detector can be easily mounted on the side wall .of a storage vesselin a horizontal or inclined disposition, or can be mounted in verticaldisposition from the top wall thereof to provide an elongated discretelevel indicator which can provide one or more zones of level indication.In a horizontal or inclined orientation, the detector can be extremelyresilient and is capable of withstanding shearing forces oftenencountered during filling or removal of material from a storage vessel.The detector can be completely sealed from its environment, with theresult that contamination does not affect electrical operation.Electrical connection to the detector is greatly simplified overconventional detectors in that only a two-wire connection need be madethereto.

The detector can also be mounted in a channel which serves to protectthe detector from forces of the material mass in which it is employed,especially in those environments where the forces can be extreme. Such amounting channel also serves to maintain the detector position inagitated or turbulant materials without detracting from itselectrical'sensitivity.

In brief, the level detector according to the invention comprises anelongated electrically conductive base strip having an insulating layerpartially surrounding the base strip and insulating the strip except foran exposed portion along the length of one surface thereof. Anelectrically conductive wire is helically wound around the base stripand insulation, the wire being spaced from the exposed portion of thebase strip by operation of the portions of the insulating layer runninglengthwise of the base strip and defining the exposed portion. Thehelically wound wire thus bridges the exposed portion of the base stripand remains electrically insulated therefrom except when an externalforce is applied to the detector.

A jacket, which may include one or more layers of material to providemoisture and abrasion resistance as required, can be employed to coverthe entire sensor and to seal the electrical system from the operatingenvironment. The conductive base strip serves as one electrical terminalof the sensor while the helically wound conductive wire serves as theother electrical terminal thereof. Each turn of the helical wirebridging the exposed portion of the base strip serves as a switchcontact, and it will be appreciated that the plurality of helicalturnsprovides redundant, electrically parallel contacts, each of whichis operative to respond to an applied force to contact the underlyingbase strip and actuate the novel sensor. The detector is connected tosuitable utilization circuitry which can provide output indications asrequired to suit particular operating requirements, or which can provideautomatic control of filling or drainage operations.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understoodfrom the following detailed description taken in conjunction.

with the accompanying drawings, in which:

FIG. 1 is an elevation view, partly in section, of a discrete leveldetector according to the invention;

FIG. 2 is a cut-away pictorial view, more particularly illustrating thesensor structure of FIG. ll;

F IG. 3 is a partly cut-away elevation view of a storage bin embodyingthe invention;

FIG. 4 is a sectional elevation view of an alternative embodiment of theinvention;

FIG. 5 is a cut-away pictorial view of the sensing element of theembodiment of FIG. 4;

FIG. 6 is a diagrammatic view of a multiple zone discrete detectoraccording to the invention;

FIG. 7 is a cut-away elevation view further illustrating the multiplezone embodiment of FIG. 6;

FIG. 8 is a diagrammatic view of an alternative multiple zone detectoraccording to the invention;

FIG. 9 is a cut-away pictorial view illustrating a sensing element ofthe embodiment of FIG. 9; and

FIG. 10 is a cut-away elevation view of a sensing element of theembodiment of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION A discrete level detectorembodying the invention is illustrated in FIG. 1 and is shown mounted tothe wall of a storage bin or other suitable storage vessel adapted tocontain material, the level of which is to be monitored. The detector 10includes an elongated sensing element 12 of flat tape-like configurationand operative in the presence of an applied force to cause actuation ofone or more of a plurality of electrically parallel redundant contactsprovided along the length thereof, as will be described hereinafter.Sensing element 12 is supported at one end within a cylindrical mountingcollar 14 for example by means of a suitable potting compound 16 such asepoxy provided within collar 14. (301- lar 14 is secured to a mountingplate 18 by means of threaded lock nut 20 cooperating with a threadedfitting 22 welded or otherwise attached to an opening provided inmounting plate 18.

A pair of electrical leads 24 connected to the redundant contacts ofsensing element 12 extend from the sensing element through the mountingcompound to a position for connection to utilization circuitry. In theillustrated embodiment, leads 24 are disposed within a connector box 26of elbow configuration threaded at one end onto mounting collar 14 andthreaded at the opposite end to an electrical conduit 28. The sensorleads can be easily channeled through conduit 28 to associatedutilization circuitry. A cover plate 30 is releads 24. The discretelevel detector 10 is disposed within the storage vessel through anopening provided in the wall thereof and, as illustrated, mounting plate18 is secured to the wall of the storage vessel by machine screws 32 orother suitable fasteners.

The elongated sensing element 12 is best described with reference toFIGS. land 2. The sensing element comprises an electrically conductivebase strip 34 having an insulating layer 36 partially surrounding basestrip 34 and insulating the strip except for an exposed portion alongthe length of one surface thereof. An electrically conductive wire 38 ishelically wound around the base strip34 and insulation 36, the helicalturns of wire 38 being spaced from the exposed portion of base strip 34by operation of the portions of insulating layer 36 extending lengthwiseof the base strip and defining the exposed portion. The helically woundwire 38 thus remains electrically insulated from base strip 34 except inthe presence of an external force applied to the sensor. In the presenceof such force, one or more of the helical turns of wire 38 will becaused to contact the underlying portion of base strip 34 and provideelectrical connection thereto and thereby cause actuation of thedetector. The sensing element is similar to that shown in U.S. Pat. No;3,153,342, assigned to the assignee of the present invention, whichutilizes a helical resistance winding and which is employed as acontinuous level sensor.

It will be appreciated that the helical turns of wire 38 provideelectrically parallel redundant contacts to effectively provide anextremely sensitive distributed switch especially adapted for actuationunder severe environmental conditions. Base strip 34 acts as oneelectrical terminal of the unique detector, while wire 38 serves as theother electrical terminal therefor. A strip of insulative material 40,for example Mylar, is provided on one end of sensing element 12 betweenbase strip 34 and the helical turns of wire 38. Strip 40 is of a widthsubstantially equal to the width of base strip 34 and serves to insulatethe base strip from the confronting turns of wire 38 to provide a meansof electrical connection to the sensing element.

Electrical connection to wire 38 is provided by means of a conductivebraid 42, disposed forexample over and under a selected number of turnsof wire 38 and which can be soldered or otherwise electrically connectedsuch as at 41 to one or more turns of wire 38. The free end ofconductive braid 42 is soldered or otherwise connected to one lead wireof the detector while a second lead wire is connected to an end of basestrip 34. A jacket 44 is provided over the electrical system of sensingelement 12 and extends along the full length thereof. A seal 46 isprovided around the distal end of sensing element 12 to prevent entry ofmoisture or other contaminants within protective jacket 44. The sensorjacket 44 can include one or more layers of material for providingmoisture, abrasion, or chemical re sistance, as required in particularoperating environments, and serves to seal the inner electrical systemfrom the operating environment as well as providing mechanicalresistance to adverse environmental conditions.

A capillary breathing tube 48 is provided within mounting collar 14 withone end thereof disposed within and connected to sensor jacket 46, theother end terminating at the inclined end of collar 14 disposed withinthe storage vessel. Tube 48 typically is filled at its open end with afiberglass filter 49 which serves to permit communication between theinterior of sensing element 12 and the interior of the storage vessel toprevent differential pressure from interfering with the sensitivity ofthe detector, while preventing contaminants which may be present withinthe storage vessel from entering the inner electrical system of sensingelement 12. It is not necessary that the sensor communicate with theoperating environment. For many purposes, the sensor can be completelyclosed to the environment, with the internal sensor volume beingsufficient to prevent internal pressure from materially affecting sensorperformance.

As illustrated in FIG. 1, the detector is disposed in the sidewall of astorage vessel and is disposed substantially horizontally therein. Inthe absence of material within the storage vessel surrounding sensingelement 12, the detector remains inactivated. When, however, storedmaterial surrounds sensing element 12, the surrounding pressure ofthestored material causes actuation of the detector by depressing one ormore turns of the helical wire into contact with the confronting basestrip, as described hereinabove, to provide detector actuation andcorresponding indication of the existence of material at the detectorlocation. Sensing element 12 is mechanically resilient in a directionparallel to the longitudinal axis of the storage vessel and is able towithstand shearing, forces along this axis, which may be of considerablemagnitude and which are often encountered during filling or removal ofmaterial from a storage vessel. In some instances it may be desirable toinstall sensing element 12 at an inclined disposition, such asillustrated in dotted outline in FIG. 1, to provide further resilienceto shearing forces encountered. This inclined disposition isparticularly useful for example when employed with solid particulatematerials, having relatively high abrasive and frictionalcharacteristics, to assist in the flow of such materials past thesensing element without adverse effect on detector operation.

Incorporation of the invention into a storage bin is illustrated in FIG.3 which shows a first discrete level detector disposed near the top of avertical storage bin 52 for providinghigh level indication, a discretedetector 54 near the bottom'portion of the bin 52 for providing lowlevel indication, and a third detector 56 disposed within the exitfunnel 58 of the storage bin to activate a low level alarm. Thedetectors 50, 54 and 56 are connected to suitable utilization circuitrywhich for example may provide alarm indications when the high leveldetector 50 is activated to indicate an overload condition, and a lowlevel indication upon actuation of detector 54 to denote that refillingof bin 52 should be made. Actuation of detector 56 can activate a lowlevel alarm and can initiate refilling of the vessel.

The detector can also be mounted vertically within a storage vessel asshown by detector 51 in FIG. 3 mounted on the top wall of vessel 52.Since the detector is responsive to and is actuated by the surroundingpressure of the material mass in the vessel, it provides a simple andeffective upper level sensor. It is not necessary that the detector beresiliently mounted, although such resilience is of great benefit inmany in-' stances, but the detector can be supported within a rigidchannel as illustrated in FIG. 4. The detector and its supportingchannel are secured at one end to a mounting adapted to provide verticalor angular attachment to a storage vessel. The mounting is a standardpipe component and permits easy installation of the invention in tanks,bins and the like using readily available hardware.

The sensor is supported within an elongated channel 132 (FIG. 5)typically formed of metal. The upper end of the sensor and its mountingchannel is secured within .a cylindrical collar 134 having threaded endportions 136 and 137. Threaded portion 136 is adapted to mate with acorresponding threaded fitting on a storage vessel, while portion 137can accommodate a suitable receptacle for electrical connection.

The mounting channel 132 is maintained within collar 134 by means of apin 138 provided through the walls of collar 134 and through channel 132and sensor 130. The sensor is further supported within the collar by asuitable potting compound 140 such as an epoxy. A disc 142 is attachedwithin collar 134 adjacent the upper end of channel 132, while a seconddisc 144 is attached within collar 134 at the upper end thereof. A venttube 146 is disposed in the manner shown within collar 134 andcommunicates at its lower end with the working environment of thesensor, and at its upper end with the space between the discs 142 and144.

A tube 148 has an end inserted within the jacket being sealed to thetube such that the interior of sensor 130 communicates with the workingatmosphere only through tube 148. The lower end of the sensor isappropriately sealed from the working atmosphere. A bag 150 formed, forexample, of polypropylene is heat sealed or otherwise secured to theupper end of tube 148, the bag being disposed within the upper portionof collar 134 between discs 142 and 1144. Bag 150 functions as a closedbreathing system and is of sufficient volume to accommodate for pressurevariations which occur during normal sensor operation. A major advantageof the closed breathing system is that the sensor can be completelyimmersed within a liquid or other fluent material being sensed withoutaffect on sensor performance. A pair of lead wires 152 isprovided asillustrated and are respectively connected to the conductive base stripand helical winding of sensor 130. Connection of the detector toutilization means can be made to suit particular requirements.

It is a particular feature of the invention that the discrete leveldetector can be fabricated as a substantially continuous element toprovide a plurality of actuation zones for providing zone levelindication and control along the entire length of the sensing element.One such multi-zoned detector is illustrated schematically in FIG. 6 andincludes an elongated sensing element 60 which comprises a conductivebase strip 62 and a helically wound conductive wire depicteddiagrammatically by winding 64. Sensing element 60 is verticallydisposed within a storage vessel and is freely suspended from a mountingcap 66. A weight 68 may be provided on the lower end of sensing element60 for stabilizing the vertical disposition of the detector. The sensingelement is easily fabricated by helically winding wire 64 around basestrip 62 and the interposed insulation, the winding being cut atselected positions, designated X, along the length thereof to provideindividual helical windings, each operative to provide a discreteactuation zone. Electrical connection is made to the respective helicalwindings by respective lead wires which can be disposed along the lengthof sensing element 60 on the side opposite to the actuation surface. InFIG. 6, lead wires 70a, 70b, 70c and 70d are connected to respectivewinding zones 64a, 64b, 64c and 64d. All lead wires pass through themounting cap 66 and typically terminate at a position outside of thestorage vessel for connection to appropriate utilization circuitry. Alead wire 72 is connected to conductive base strip 62 to provide acommon terminal for each winding of the multiple zone detector. Thewinding 64 is insulated from base strip 62 by an insulative strip 63disposed therebetween, except at portions 65 within the length ofrespective discrete windings 64a 64d. These uninsulated portions 65define an actuation zone for each winding wherein each winding can beactuated by the pressure of a surrounding medium.

Connection of a lead wire to respective winding zones of the elongateddetector can be accomplished for example in a similar manner as shown inFIG. 2. A flexible conductive braid is secured under a plurality ofturns of winding 64, with lead wire 70 connected to one end of thebraid, for example by soldering. Alternatively, the lead wire can besoldered directly to one or more turns of winding 64.

The multi-zone sensor is further depicted in FIG. 7. Helical winding 88has been broken at points 90 and 92 to provide separate discretedetectors 82, 84 and 86, only a portion of detectors 82 and 86 beingillustrated. The active actuation zone of each discrete detector such aszone 85 of winding 84, is defined by strips of insulative material 94disposed between the turns of helical winding 88 and the underlyingconductive base strip.

An alternative embodiment of a multiple zone discrete level detector isillustrated in FIG. 8 and includes a plurality of horizontally orangularly disposed sensing elements connected to and supported by afreely hanging vertically disposed support strip. Referring to FIG. 8,there is shown a vertically disposed conductive strip 96 depending fromand insulated from a mounting cap 98 and having a stabilizing weight 100provided on the lower end thereof. A plurality of horizontally disposedelongated sensing elements 102 is supported by strip 96 and each isspaced along the length thereof at selected intervals to providediscrete level indicating positions for actuation by the pressure of asurrounding stored material. The base strip of respective sensingelements 102 is electrically connected to conductive strip 96 which, inturn, is connected to a common lead wire 104. Respective lead wires 106are connected to the helical actuation winding of sensing elements 162to provide individual electrical connection to respective discretedetectors.

Construction of an individual sensing element 102 is illustrated moreclearly in FIGS. 9 and 10. The base strip 108 of sensing element 102 ismechanically joined to vertical conductive strip 96, for example, bymeans of a hollow rivet 110 provided near the upper end of base strip108 and by a winding of tape 112 wound around strips 108 and 96 near thebend in strip 108. A strip 114 of insulative material such as Mylar isdisposed between a portion of conductive winding 116 and the underlyingconductive base strip 108 and extends beneath tape 112 to a positionnear hollow rivet 110. Electrical connection is made to winding 116 by awire braid 118 interposed between the turns of the winding and extendingpart way onto underlying insulative strip 114. A lead wire 120 disposedalong the back side of strip 96 passes through hollow rivet and issoldered or otherwise connected to the conductive braid 118.

As described hereinabove, an activation zone 122 is defined by theinsulative strip 114 and a second insulative strip 124 provided at thedistal end of the sensing element between the conductive base strip andthe confronting turns of the helical winding 116. A protective jacket126 is provided over the electrical system for isolation thereof fromthe working environment, as described. In this embodiment, pressuredifferentials which may exist between the sensor interior and theworking environment are accommodated by capillary breathing tube 128disposed at the distal end of the sensing element and communicatingbetween the sensor interior and the ambient environment.

The detector can be connected to any known utilization circuitry to suitparticular operating requirements. In general, the detector leadsareconnected to a relay which is operative upon actuation of the detector,the

relay contacts being employed to activateparticular utilizationcircuitry.

Various modifications and alternative implementations will occur tothose versed in the art and it is not intended to limit the invention bywhat has been particularly shown and described. 7

What is claimed is:

1. A level detector comprising: 7

an elongated electrically conductive base strip;

electrical insulation means disposed around and insulating said basestrip with the exception of a full length exposed portion on one sidethereof;

an electrically conductive wire helically wound around said base stripand insulated therefrom by said insulation means, the portions ofconductive wire bridging said insulation means acting as electricallyparallel redundant contacts;

a flexible jacket enclosing said base strip, said insulation means andsaid conductive wire;

means electrically connected to said base strip and said conductive wireand adapted for connection to utilization means; and

means attached to one end of said detector for mounting said detector toa storage vessel in a position to sense the presence of surroundingmaterial therein;

said detector being responsive to the pressure of surrounding materialto cause contact of one or more turns of said helical wire with saidconductive base strip thereby to indicate the presence of material atthe detector position;

and wherein said mounting means includes a cylindrical pipe fittingdisposed around one end of said detector and rigidly secured thereto;

sealing means provided on the distal end of said de tector for sealingsaid distal end from the working environment;

a closed breathing system including a tube having one end disposedwithin the interior of said detector and an opposite end secured to abag disposed within said fitting; and

a vent tube disposed within said fitting and communicating between theupper portion thereof and the working environment of said detector.

2. A level detector according to claim 1 further including a rigidchannel substantially coextensive with and supporting said detector andhaving an end portion rigidly secured within said fitting.

3. A level detector for detecting the level of material at distinctzones along the length thereof, said detector comprising:

a plurality of distinct detectors each including an elongatedelectrically conductive base strip; electrical insulation means disposedaround and insulating said base strip with the exception of a fulllength exposed portion on one side thereof;

an electrically conductive wire helically wound around said base stripand insulated therefrom by said insulation means, the portions ofconductive wire bridging said insulation means acting as electricallyparallel redundant contacts;

a flexible jacket enclosing said base strip, said insulation means andsaid conductive wire;

an elongated conductive support strip adapted for vertical dispositionwithin a storage vessel; each of said distinct detectors being connectedto said conductive support strip at selected intervals along the lengththereof with the conductive base strip of each of said distinctdetectors being electrically attached to said conductive support strip;

each of said distinct detectors being disposed in angular relation tosaid conductive support strip; and

means respectively connected to the helically wound wire of each of saiddistinct detectors for providing respective electrical connection toeach of said distinct detectors, said conductive support strip providinga common electrical connection for all of said distinct detectors.

4. A level detector according to claim 3 including a plurality of venttubes each attached to the distal end of a respective distinct detectorand each communicating between the interior of its detector and theworking environment.

5. A level detector comprising:

an elongated electrically conductive base strip;

electrical insulation means disposed around and insulating said basestrip with the exception of a full length exposed portion on one sidethereof; an electrically conductive wire helically wound around saidbase strip and insulated therefrom by said insulation means, theportions of conductive wire bridging said insulation means acting aselectrically parallel redundant contacts; a flexible jacket enclosingsaid base strip, said insulation means and said conductive wire;

means electrically connected to said base strip and said conductive wireand adapted for connection to utilization means; and

means attached to one end of said detector for mounting said detector toa storage vessel in a substantially horizontal position therein to sensethe presence of surrounding material therein;

said detector being responsive to the presence of surrounding materialto cause contact of one or more turns of said helical wire with saidconductive base strip thereby to indicate the presence of material atthe detector position.

6. A level detector comprising:

an elongated electrically conductive base strip;

electrical insulation means disposed around and insulating said basestrip with the exception of a full length exposed portion on one sidethereof;

an electrically conductive wire helically wound around said base stripand insulated therefrom by said insulation means, the portions ofconductive wire bridging said insulation means acting as electricallyparallel redundant contacts;

a flexible jacket enclosing said base strip, said insulation means andsaid conductive wire;

means electrically connected to said base strip and said conductive wireand adapted for connection to utilization means; and

means attached to one end of said detector for mounting said detector toa storage vessel in an inclined position therein to sense the presenceof surrounding material therein;

said detector being responsive to the presence of surrounding materialto cause contact of one or more turns of said helical wire with saidconductive base strip thereby to indicate the presence of material atthe detector position.

7. A level detector for detecting the presence of material at distinctzones along the length thereof, said detector comprising:

an elongated electrically conductive base strip;

electrical insulation means disposed around and insulating said basestrip with the exception of a full length exposed portion on one sidethereof;

an electrically conductive wire helically wound around said base stripand insulated therefrom by said insulation means, said helical wirebeing severed at selected points along the length thereof to provide aplurality of distinct helical coils;

a plurality of electrical leads each lead being connected to arespective coil;

an electrical lead connected to said base strip;

said leads being adapted for connection to utilization circuitry;

insulation means disposed between said helically wound wire and saidconductive base strip at selected portions along the length of saiddetector to define at regions between adjacent insulation means distinctactive'zones;

a flexible jacket enclosing said base strip said insulation means andsaid wire; and

means for mounting said detector to a storage vessel in a verticalposition to sense the presence of surrounding material therein;

said detector being responsive to the pressure of surrounding materialto cause contact of one or more turns of said helical wire with saidconductive base strip, the actuation of a selected helical coilindicating the presence of material at the detector zone defined by thatcoil position.

8. A level detector comprising:

an elongated electrically conductive base strip;

electrical insulation means disposed around and insulating said basestrip with the exception of a full length exposed portion on one sidethereof;

an. electrically conductive wire helically wound around said base stripand insulated therefrom by said insulation means, the portions ofconductive wire bridging said insulation means acting as electricallyparallel redundant contacts;

a flexible jacket enclosing said base strip, said insulation means andsaid conductive wire;

means electrically connected to said base strip and said conductive wireand adapted for connection to utilization means; and

means attached to one end of said detector for mounting said detector toa storage vessel in a position to sense the presence of surroundingmaterial therein;

said detector being responsive to the presence of surrounding materialto cause contact of one or more turns of said helical wire with saidconductive base strip thereby to indicate the presence of material atthe detector position;

and wherein said mounting means includes a collar attached to andsupporting said detector on one end thereof;

a capillary breathing tube including a filler therein disposed withinsaid collar and communicating be tween the interior of said detector andthe working environment;

sealing means provided on the distal end of said detector for sealingsaid distal end from said environment;

and wherein said electrical connection means includes a conductive braidelectrically and mechanically connected to one or more turnsof saidconductive wire at the mounting end of said detector;

insulation means disposed between said wire and said conductive basestrip for insulating said conductive braid from said base strip;

a first lead wire having one end connected to said conductive braid andan opposite endadapted for connection to utilization means; and

a second lead wire having one end connected to said conductive basestrip and an opposite end adapted for connection to said utilizationmeans.

9. A level detector comprising:

an elongated electrically conductive base strip;

electrical insulation means disposed around and insulating said basestrip with the exception of a full length exposed portion on one sidethereof;

an electrically conductive wire helically wound around said base stripand insulated therefrom by said insulation means, the portions ofconductive wire bridging said insulation means acting as electricallyparallel redundant contacts;

a flexible jacket enclosing said base strip, said insulation means andsaid conductive wire;

means electrically connected to said base strip and said conductive wireand adapted for connection to utilization means; and

means attached to one end of said detector for mounting said detector toa storage vessel in a position to sense the presence of surroundingmaterial therein; a

said detector being responsive to the pressure of surrounding materialto cause contact of one or more turns of said helical wire with saidconductive base strip thereby to indicate the presence of material atthe detector position;

and wherein said mounting means includes a cylindrical pipe fittingdisposed'aroundw one end of said detector and rigidly secured thereto;

sealing means provided on the distal end of said detector for sealingsaid distal end from the working environment; a

a closed breathing system including a tube having one end disposedwithin the interior of said detector and an opposite end secured to acontainer disposed within said fitting; and

a vent tube disposed within said fitting and communicating between theupper portion thereof and the working environment of said detector.

